The present invention relates to an ion source suited for ionizing a sample existing in a liquid to introduce the ionized sample into a mass spectrometer, and a mass spectrometer using the ion source.
Using capillary electrophoresis (CE) or the liquid chromatograph (LC) it is easy to separate a sample existing in a solution but difficult to identify the kinds of samples separated. On the other hand, a mass spectrometer (MS) can identify the separated sample with high accuracy. Thus, when it is intended to separate and analyze a plurality of biological substances dissolved in a solvent such as water, there is generally used capillary electrophoresis in combination with a mass spectrometer (CE/MS) or liquid chromatograph in combination with a mass spectrometer (LC/MS) which is constructed by combining the capillary electrophoresis or the liquid chromatograph with the mass spectrometer.
In order to analyze the sample, which is separated by the capillary electrophoresis or the liquid chromatograph, using the mass spectrometer, it is necessary to transform the sample molecules in the solution into gaseous ions, i.e. particles or gaseous materials. A conventional technique for producing such ions is known as the ion spray method (as is disclosed on pp 2642 to 2646, Analytical Chemistry, Vol. 59 (1987)). In the ion spray method, the gas is introduced along the outer circumference of a capillary, and a high voltage (e.g., 3 to 6 kV) is applied between the capillary to be fed with the sample solution and an aperture (e.g., the sampling orifice) for introducing the ions into the mass spectrometer, so that an intense electric field is established at the capillary tip. By the electrospray phenomenon established by that construction, there are produced fine charged droplets, which are evaporated by the aforementioned gas to form gaseous ions, i.e. gaseous particle or gaseous materials. The ions thus formed are introduced via the sampling orifice into the mass spectrometer so that they are mass-analyzed. The aforementioned gas promotes the atomization of the charged droplets and suppresses the discharge at the tip of the capillary.
Another conventional technique is known as the electrospray method of ionizing a solution with no gas flow at a flow rate of 10 xcexcL (i.e., microliters)/min. to the capillary (as disclosed on pp. 4451 to 4459, Journal of Physical Chemistry, Vol. 88 [1984]). The electrospray method is different from the ion-spray method but has the same ionization principle as that of the ion-spray method.
A further conventional technique is known as the atmospheric pressure chemical ionization method (as disclosed on pp. 143 to 146, Analytical Chemistry, Vol. 54 [1982]). In the atmosphere pressure chemical ionization method there is disposed in the vicinity of the tip of the heated capillary an electrode for generating a corona discharge to ionize the volatile molecules sprayed under atmospheric pressure.
The various conventional spray ionization methods described above in order to achieve a high ionization efficiency, it is necessary to form fine charged droplets having a diameter no more than about 10 nm.
In the conventional techniques described above, a high voltage is applied around the sampling orifice. This application makes it necessary to avoid an electric shock, thus causing a problem that the instrument has a complicated structure. Since the high voltage is applied to the capillary tip in the CE/MS, a higher voltage has to be applied so that the electrophoresis of the sample may be effected in the capillary electrophoresis instrument.
Moreover, the electrospray phenomenon is so seriously influenced by contamination at the tip of the capillary and on the surface of the sampling orifice that once the spray of the sample solution is interrupted the electrospray method or the ion spray method detects different ion intensities with a poor reproducibility even if the spray is reopened under the identical conditions. In order to maximize the ion intensity detected, therefore, the troublesome operations of finely adjusting the capillary position or cleaning the capillary tip and the sampling orifice surface are required each time the spraying operation is reopened. As a result, the structure of the instrument is so complicated for avoiding electric shock that the operations are obstructed.
In the conventional techniques described above, moreover, the sample solution has to be mixed with volatile molecules such as alcohol or ammonia as the solvent. It has been empirically known that no electrospray phenomenon occurs when the solvent used has a low electric conductivity, and that the electric conductivity of the sample solution has to be within 10xe2x88x9213 to 10xe2x88x9215 xcexa9cmxe2x88x921 so as to establish the electrospray phenomenon. Thus, there arises a problem that so long as those conditions are not satisfied, the electrospray phenomenon does not stably occur to limit the selection of the solvent.
Further, since a high voltage is applied between the capillary and the sampling orifice, a discharge may occur around the ion source to make it difficult to use an inflammable solvent. If the kind of solvent to be used is thus limited, the substance to be measured may be unable to be separated by the capillary electrophoresis or the liquid chromatograph.
The object of the present invention is to provide an ion source that can be safely and easily operated and a mass spectrometer instrument which is capable of producing ions stably and analyzing a sample with high sensitivity and with an excellent reproducibility by using the ion source.
Another object of the present invention is to provide an ion source, which can use a wide range of solvents in the capillary electrophoresis or liquid chromatograph, and a mass spectrometer instrument using the ion source.
The present invention includes an ion source having an ion source body for forming a gas flow around the outer circumference of the tip of a capillary to be fed with a sample solution, so that the gas is sprayed around the outer circumference of the tip into the air to ionize the sample solution. In the present invention the Mach number is determined by the flow velocity of the gas and its sonic velocity to be at least within a range around 1. Moreover, the ion source body is constructed to have a gas inlet port for introducing the gas and an orifice for spraying the gas, into which is inserted the tip of the capillary so that the gas is sprayed from a small volume formed between the outer circumference and the inner circumference of the orifice. The present invention may alternatively include a mass spectrometer instrument using the aforementioned ion source.
The characteristics of the ion source of the present invention will be described in more detail in the following. The ion source includes a capillary for feeding a sample solution into the air, and an ion source body having an orifice for receiving the tip of the capillary and forming a gas flow along the outer circumference of the capillary to the tip of the capillary. A characteristic value F/S dictating that the gas flow is within a predetermined range is determined by a flow rate F of the gas reduced into the standard state (i.e. standard conditions) (20xc2x0 C., 1 atm) and the cross section of a cross section normal to the center axis of the orifice, whereby the sample solution fed into the air is ionized in the vicinity of the tip of the capillary by the gas flow. The desired predetermined range of the aforementioned characteristic value F/S is 200 m/s to 1000 m/s. In order to ionize the sample efficiently, the aforementioned characteristic value F/S is preferably set to 350 m/s to 700 m/s and more preferably set to 500 m/s to 600 m/s. Here, the value F/S has the same dimensions as those of a velocity but is different from the actual velocity of the sprayed gas. The flow rate F is a value which is reduced from the flow rate of the sprayed gas in the standard state. The actual sprayed gas has a higher pressure than 1 atm. Incidentally, the flow rate of the sample solution is set to 1 xcexcL (i.e., microliters)/min. to 200 xcexcL (i.e., microliters)/min.
By the gas sprayed from the small volume at the tip of the capillary, fine charged droplets of the sample solution are formed at the capillary tip. When the Mach number of the gas flow approaches 1, finer charged droplets are formed. By the sprayed gas, the solvent is gasified from the formed charged droplets to produce gaseous ions, i.e. gaseous particles or gaseous materials. The ions thus produced can be introduced into and analyzed by the mass spectrometer.
When the characteristic value F/S of the sprayed gas flow at the capillary tip exceeds a certain value, the sample solution introduced into the capillary is broken into charged droplets of various sizes at the capillary tip. The extremely fine charged droplets of less than at least 100 nm are easily desolved (or dried). Even the neutral sample molecules may be bonded to protons or sodium ions in the extremely fine droplets to produce quasi-molecular ions so that the ions can be analyzed by the mass spectrometer instrument.
The conditions for determining the size of the droplets to be formed at the capillary tip are essentially the characteristic value F/S or the Mach number of the sprayed gas flow. In the production efficiency of the extremely fine droplets, there are other factors to be considered. In other words, the pressure difference between the solution surface and the volume surrounding the capillary tip has to be larger than a certain value. By reducing the capillary wall thickness to 100xcexc or less, the production efficiency for the extremely fine droplets can be enhanced.
Moreover, the reproducibility of the ionization conditions can also be enhanced by aligning the center axis of the capillary with the center axis of the orifice of the ion source body to make the gas velocity uniform at the tip of the capillary so that the sprayed gas containing the droplets of the sample solution may be axially symmetric.
If the characteristic value F/S of the gas flow is constant, the droplets of the sample solution have substantially the same distribution of their size and have no substantial relation to the gas flow rate F and the cross section S of the small volume for spraying the gas. Empirically, it is sufficient that the gas flow rate F be 0.5 liters/min. or more. The material of the capillary and the potential to be applied to the capillary have no substantial influence upon the size of the droplets to be produced from the solution.
According to the present invention, the ions can be efficiently produced from the sample solution by the sprayed gas while grounding the potentials of the individual portions such as the capillary constituting the ion source to the earth. As a result, the ion source can have its structure made simpler and its operability and safety enhanced better than those of the conventional ionization method. Moreover, when the ion source of the present invention is applied to the capillary electrophoresis instrument to constitute the CE/MS, the tip of capillary can be grounded to earth, as described above, and the capillary electrophoresis can independently apply a potential thereto thus greatly simplifying its entire construction and its operation and drastically improving its operational safety.
In the conventional ionization method, such as electrospray and ion spray methods, the ionization is highly influenced by contamination around the capillary and the sampling orifice. In the sonic spray method of the present invention for producing the ions from the sample solution by the sprayed gas, on the contrary, the ionization is not influenced by contamination around the capillary and the sampling orifice.
In the conventional ionization the ion intensity to be detected is highly influenced by contamination around the sampling orifice and at the capillary. In the sonic spray method of the present invention, on the contrary, the ion intensity to be detected is influenced neither by contamination around the capillary nor by contamination around the sampling orifice so that the sample can be detected with high sensitivity and with an excellent reproducibility. In short, the capillary tip and the ion source body are arranged in optimum positions so that the ions can be produced and detected from the sample solution with an excellent reproducibility and in a high efficiency.